Recently developed continuous circulation sand filters not only filter introduced water by using sand, and but also wash the sand. Such a continuous circulation sand filter is illustrated in FIGS. 1 and 2.
Referring to FIGS. 1 and 2, the continuous circulation sand filter includes a raw water inflow tube 1, a raw water distribution tube 7, a sand filtering layer 8, and a treated water outflow tube 2. Raw water introduced through the raw water inflow tube 1 is uniformly dispersed in the sand filtering layer 8 through the raw water distribution tube 7, and While the introduced raw water passes through the sand filtering layer 8, a foreign substance is removed from the introduced raw water to produce treated water that is discharged through the treated water outflow tube 2.
Contaminated sand located in the lower part of the continuous circulation sand filter is moved upward through an air lift tube 4 by compressed air generated in a compressed air supply part 5 and moved through an air inflow tube 6. A sand washing device 10 is disposed on the upper part of the air lift tube 4 as illustrated in FIG. 2.
Sand, arriving at the sand washing device 10, is moved downward through a bellows tube 11 from a washing barrel 15, and is stacked on the upper part of the sand filtering layer 8. A foreign substance removed from the sand is discharged through a washing water outflow tube 3.
Since the bellows tube 11 is disposed under the washing barrel 15 of the sand washing device 10, when sand moving in a zigzag shape temporarily stays within the bellows tube 11, treated water is moved from the lower end of the bellows tube 11 to the upper end thereof, so as to remove a foreign substance from the sand.
As such, since transfer water, moving upward together with sand through an air lift tube of a continuous circulation sand filter, is discharged together with sand washing water through a washing water outflow tube, the generation of sand washing water is increased. In addition, although the continuous circulation sand filter includes a bellows tube to sufficiently remove a foreign substance, while a foreign substance is removed, the foreign substance may flow down together with sand from a washing barrel and be undesirably discharged together with filtered water through a treated water outflow tube. In addition, while sand is stagnant in a sand washing device, the washing barrel of the sand washing device and the bellows tube may be clogged with sand.
Japanese Patent Publication Nos. 2003-200185 and 2004-255261 disclose an apparatus that moves a filtering material upward through an air lift tube to thereby remove a foreign substance from the filtering material. The apparatus includes a separation device at the upper end of the air lift tube to separate water, a solid formed through a biological treatment, and the filtering material moving upward through the air lift tube. The separation device includes a screen that traps the filtering material erupting from the air lift tube and that returns the trapped filtering material into the apparatus. The water and the solid, passing through the screen, are discharged through a washing drain tube. However, since such a separation device separates a filtering material first, and discharges water and solid together, an amount of contaminated discharged water is increased.